tests: implement integration harness

[totocaster]

Summary

Docs still reference a future integration test suite under tests/integration/. Build that harness so CLI, daemon, and MCP flows have end-to-end coverage.

Draft Plan

• Define a tests/integration/ crate that spins up a temporary copy of tests/fixtures/test-vault.
• Add helpers to launch the daemon with embeddings disabled and wait for initial indexing.
• Exercise CLI commands (search, notes, graph, index status) against the running daemon to assert happy paths.
• Cover MCP stdio and HTTP health/authentication flows in integration scenarios.
• Document harness expectations and usage in tests/README.md.

Definition of Done

• Integration tests run under cargo test --all-targets.
• CI executes the suite in the default workflows.
• Docs no longer describe the integration harness as "to be created".